meson: opengl: fix enabled_gl_apis in pkg-config file
requested to merge tpm/gst-plugins-base:meson-opengl-fix-enabled-gl-apis-in-pkgconfig-file into 1.14
Make consistent with what autotools puts into enabled_gl_apis variable. Autotools puts 'gl' in there instead of 'opengl'.
This would cause problems when building -bad glmixers plugin in meson against a -base that was built with autotools.